I70 THE BIOLOGY OF TWINS 



Nurture." In a subsequent paper (1892) he shows his 

 continued interest in this problem by his remark: 

 "It may be mentioned that I have an enquiry in view 

 which has not yet been fairly begun, namely: to deter- 

 mine the minutest biological unit that may be 

 hereditarily transmissible. The minutiae in the finger- 

 prints of twins seem suitable objects for this purpose." 

 Wilder in his paper on "Duplicate Twins and 

 Double Monsters" follows up this clue and presents 

 many important facts as to the close resemblance 

 between twins in the patterns of the friction ridges 

 in palms and soles. The conclusion reached is as 

 follows : 



The influence of the germ-plasm and its mechanism [i.e., 

 the direct control exercised by heredity] is exerted upon the 

 friction-skin surfaces only so far as concerns the general con- 

 figuration, i.e., the main lines, the patterns, and other similar 

 features; the individual ridges and their details [minutiae] are 

 apparently under the control of individual mechanical laws to 

 which they are subjected during growth. Have we then arrived 

 at the limit of the control of the predetermining mechanism 

 beyond which mechanical laws are alone operative, and is it 

 then possible to hold that the modifications in the latter field are 

 the results of individual experience, and that they are similar 

 in the various members of a given species solely because of s imil ar 

 environment? To these and similar questions we can give no 

 answer at present; yet it seems likely that in general in the pahn 

 and sole markings, not ofily in man but in other mammals as 

 well, we have a set of easily observed and very significant data 

 which may yield important results to future investigators. 



Data similar to that on the friction-ridge patterns 

 of human twins are afforded by a study of scute and 

 band doubling in armadillo quadruplets. Just as 

 there is in human twins usually a striking general 



